Differential protection means



Feb. 21, 1939. R. L WARD DIFFERENTIAL PROTECTTON MEANS Filed Feb. lO, 1936 3 Sheets-Sheet 2 Feb. 21, 1939.l

R. l. WARD 2,147,781

DIFFERENT IAI.. PROTECT TON MEANS Filed Feb. 10, 1936 3 Sheets-Sheet 3 Patented Feb. 21, 1939 PATENT OFFICE 2,147,181' y nmFEaENTIAL raorEc'rroN MEANS neben I. ward, chicago, m.

Application February 10, "1936, Serial No. 63,135

z claims. (crus-294) v This invention relates to means for detecting and protecting against certain types of faults to which alternating" current systems are subject, such as short circuits, grounds or other faults which produce unbalanced conditions in distribution lines or the like.

The particular invention described herein is a' continuation in part of that disclosed in my copending application, Serial No. 573,754, filed November 9, 1931, now issued as Patent No. 2,047,343, dated July 14, 1936, and is directed particularly to the protection of lines against transformer faults or unbalanced conditions therein, and to the diiferential protection of sections of alternating current systems.

For this type of protection, the present alternating current distribution systems require apparatus that must be highly sensitive to unbalanced conditions and the like, being directional at approximately one percent normal voltage and operable in a fraction of a second under overload. There is considerable difficulty in designing apparatus capable of this sensitivity and maintain- U ing it operable at such sensitivity at all times.

I have devised a relayarrangement lwherein the sensitive element comprises a high vacuum tube, providing a degree of sensitivity combined with reliability hitherto unknown. I prefer to connect the grid and plate circuits of the tube in such manner that the grid prevents any substantial or operative'current flow in the plate circuit under normal conditions, and permits operative plate current flow upon occurrence of a fault or unbalanced condition.

It is one of the objects of the present invention to provide a control relay in the form of a three element vacuum tube wherein a certain alternating voltage is applied to the grid, a certain alternating voltage is applied to the plate, and the plate circuit serves as an indication of the variations in the relative phase angle between .the plate and grid voltages. The two voltages may be functions ofthe currents flowing in different parts 'of the system whereby the relay may be used for dierential line protection, or for protection against faults which result in a relative phase change of the currents or voltages in different parts of the system, orfor protectionl against faults occurring within a transformer or the like.

The present invention may be employed as a protection against internal faults in power transformers such as short-circuits, faults to ground or such conditions where the power output does not balance the power input (neglecting normal losses, magnetizing current etc.). The protection is also effective for faults where the currents in the phase leads of a 3phase transformer do not balance resulting in neutral current flow. The present invention may be such as to clear a 5 fault instantaneously or may be modified to clear the fault after a predetermined time depending upon the characteristics necessary for the particular type of fault protection.

The invention is also applicable to the protection of generators. The generator protection is of the same general character as that to be described in connection with the transformer protection, and embodies the same features and principles of the invention.

If desired, the protection of the transformer may be so designed as to provide for separate control circuits for each transformer winding, with an individual control tube responsive to an unbalanced condition in the respective winding. The timing circuit may also be included in this modification, if so desired. l

I have also provided for a differential control system responsive to relative phase angle change of the currents on the primary and secondary 25 sides of the transformer, in order to detect faults in the transformer and to protect the external distribution system against such faults.

The attainment of the above and further objects of the present invention will be more ap- 30 parent from the following detailed description which, taken in commotion with the accompanying drawings, will disclose to those skilled in the art the particular construction and operation of preferred forms of the present invention. 35:

In the drawings: n

Figure 1 is a circuit diagram illustrating the control system applied for operation responsive to the occurrence of an internal fault in a transformer;

Figure 2 is a circuit diagram illustrating a pilot wire protecting arrangement embodying the principies of my invention;

Figure 3 is a modication of thesystem shown in Figure 2;

Figure 4 is a circuit diagram illustrating the relay protective system connected internally of a transformer;

Figure 5 is a modification of the circuit shown in Figure 4;

Figure 6 shows a modified transformer connection;

Figure '7 shows the protection circuit for one of the transformer connections shown in Figure 6; 5'5

Figure 8 is a modification of the circuit shown in Figure 7; and

Figure 9 is a modification of the transformer connection shown in Figure 6.

Reference may now be had more particularlyv to Figure 2, wherein I have shown myv invention applied to a section of a three phase power line for isolating the section upon the occurrence vof a fault'therein. The section to which the protection is applied is indicated at 50, and forms a connecting link between the two parts of the system indicated as a power line 5| and a power line 52. Circuit breakers 53 and 54 at the two ends of the section 50 connect this section with the rest of the system. 'I 'he apparatus at the left hand end of the section 50 comprises three current transformers 6|), 6| and |52, one for each phase of the system. 'I'he secondaries of the f three current transformers are connected in parallel with one another and in parallel with the primary 64 of a special pilot transformer- 65. When the load carried by the three conductors of the section 5|) is balanced, no current will ow through the primary 64 of thetransformer 55.

The secondary 66 of the transformerv 65is con-- nected to supply voltage to the plate of a high vacuum tube 10 of a construction similar to that of the tube previously described. 'I'he plate is indicated at 1|, the cathode at 12, the cathode being grounded through a suitable resistance 13.

A relay 15 is connected in the plate circuit of the tube 10, the relay controlling the tripping of the circuit breaker 53 through the trip coil 56.

At the right end of the section 50 there is provided a set of apparatus similar to that at the left hand end of the section, and similar reference numerals have been used to indicate similar parts of the two ends of the section, the reference numerals for the part at the right end of the section being primed. The secondary of the transformer 65 is provided with a tap 80 which supplies voltage to the grid of the tube 10. A

.similar tap on the secondary of vtransformer 65' supplies voltage to the grid of the tube 10. For this purpose two pilot wires 8| and 82 are provided.

Under normal operation when there is a balanced load upon the three conductors A, B and C of the section 50 no current fiows through the primaries of the transformers 65 and B5', and hence there is no voltage upon the plate or the grid of the tube. The tripping relays 15 and 15 therefore remain unenergized. Under certain fault conditions current does flow through the primaries 64 and 64' resulting in an operation of therelay system to isolate the section of the line.

An explanation will now be given on the operation of this system upon the occurrence of fault at various places in the system and under various conditions. Assume that a ground fault occurs on the C phase conductor at the point marked X. Assume further that power is being sent to the fault from both the line 5| and the line 52. The unbalanced current in the current transformers 60, 6I and 62 will ow through the primary of the transformer 65 and will induce a voltage Vin the secondary thereof, thus placing an alternating voltage upon the plate 1| of the tube 10, and, through the conductor' 82, placing an alternating voltage upon the grid of the tube 10. At the same time power is being fed to the fault from the line 52 thereby producing a similar condition in the apparatus at the right hand end of the section 50 whereby an alternating voltage is impressed on the plate of the tube 10' and,-through the pilot conductor 8|, upon the grid of the tube 1B through ground. Since the lines 5| and 52 are interconnected as part of a network the voltages on these two lines will be in phase with one another, hence the current through the primaries 54 and 64' will be in phase with one another. From this it follows that the voltage on the grid of the tube 10 as supplied by the secondary 66 of the transformer 65' will be substantially in phase with the voltage on the plate 1| as supplied by the secondary of the transformer 65. A' similar phase relationship exists between the voltage on the grid and plate of the tube 10. The grids will therefore permit current to ow through the respective plate circuits and bring about the energization of .the relays 15 and 15 which, upon closing, complete the circuits through the relays 55 and 56' for tripping the circuit breakers 53 If power were being fed to the ground fault from only one end of the line the operation would be similar to that as above set forth. Assume that the line 5| 'is the power line and the line 52 extends to a load and isnot connected-to any other source of power. Fault current through the C phase conductor will-result in acurrent flow Vthrough the primary 64 of transformer 65 inthe Athe line 52 is'later energized and the fault still remains on the C phase, the circuit breaker 54 will be tripped as a result of a similar sequence of operations.

If, at the time of occurrence of the fault at .the point marked X there is. no load upon the conductors 52, Vor if the equivalent results from the fact thatvthe circuit breaker 54 happens to be opened at the time, thevoperation of the system will not be adversely'eected.' Under such conditions there is an alternating voltage impressed upon the plate of the tube 10 and an alternating voltage impressed upon the grid of the tube 10. There is no voltage upon the grid of the tube 10 and there is no voltage upon the plate of the tube lll'. The tube 10' therefore does not produce an operation of the relay 15. On the other hand plate current does flow through the plate circuit of the tube 10, notwithstanding the absence of the grid potential due tothe fact that the tube here employed is so designed that it is not necessary tohave a grid voltage in order to initiate current flow in the cathode-plate circuit. The grid is effective only to prevent the plate current .flow when the grid isat a negative potential.

,the plate circuit of the tube when there is plate voltage and there is noimpediment from the grid. Thus if the grid voltage is in phase with the plate voltage, there will be a plate current ow during each half cycle that the plate voltage is positive. The same condition will prevail when there is no voltage on the grid. However, if the grid and the plate are substantially 180 degrees out of phase, then the grid will prevent plate current flow. It is to be noted that the current transformers at the two ends of the section 50 are oppositely connected so that when the current is flowing in the same direction at the two ends of the section the unbalanced current,if

any, in the primaries of the transformers 65, will be in opposite directions whereas if the directions of iiow at the two ends of the section are in opposition, then the resulting current in the primaries of the transformer 65, 65' will be in the same direction.

When a fault occurs externally of the section 50 but so related to the section that the unbalanced fault current flows therethroughit is not necessary, nor desirable, to open the circuit breakers 53-'54 since the fault, being external ofthe section under consideration, should be cleared by the protective apparatus provided at the fault, thus permitting as much of the system as possible to remain in service. To show that the circuit breakers 53-54 are not tripped under such circumstances let us assume that a ground fault occurs on one of the phase conductors of the line 52 and is supplied with power from the line 5| by way of the section 50. Assume that-the fault is on the C-phase conductor. An unbalanced current will ow through the section 50, the instantaneous direction of flow being the same at the two ends of the section. A large current will ow through the current transformer 62 to ground by way of the primary 64 and a similar current will flow through the transformer 62 to ground by way of the primary of the transformer 65'. It is to be noted that the secondaries of the two current transformers are oppositely connected with respect to the primaries of the associated pilot transformers 65 and 65. 'I'herefore the current flowing through the primaries of the transformers 65 and 65' will be approximately 180 degrees out of phase. Since each Yof the two tubes 10 and 1| is supplied with plate voltage from one of the pilot transformers and grid voltage from the other pilot transformer it follows` that the plate and grid voltages of the two tubes will be substantially 180 degrees out of phase and hence the grids will prevent the flow of current through the respective plate circuits. Therefore the relays 15 and 15' will not operate and the circuit breakers will not be tripped.

Referring now to Fig. 3, wherein I show my invention as applied to a somewhat different protective arrangement, this system is identical to that shown in Figure 2 except for the fact that two additional transformers 90 and 9| are provided at one end of the section and two current transformers 90 and 9| are provided at the other end of the section.v The circuit shown in Figure 3 is adapted to be connected in a system such as conductors of the line 50 of Figure 3. This is true whether the overload is brought about by a phase to ground fault, a two phase fault, or a three phase fault, even though the three phase fault results in a balanced currrent flow through the three phase conductors of the line 50. A further description as to why this takes place is to be found in the patent to Le Clair and Gross, No. 1,919,231, of July 25, 1933. This voltage will induce a current ow through the primary of the transformer 65 with the result that a voltage will be induced in the secondary of this transformer. A similar action takes place at the other end of the section 50. The actionof the two tubes in the system shown in Figure 3 will be identical to that of the tubes shown in the system of Figure 2. When the voltages on the secondaries of the transformers 65 and 65 are in phase the grids of the respective tubes will permit a current flow Whereas when the voltages are 180 degrees out of phase the grids will prevent aplate current flow. It is to be noted that when the overload current is in the same direction at the two ends of the section 50, indicating that the fault is external of the section, the voltages of the secondaries of the transformers 65 and 65 will be in opposition, whereby no current flows through the plate circuit, whereas when the currents flow in the same relative directions at the two ends of the line, indicating a fault between the two ends of the line, the voltages at the secondaries of the transformers 65 and 65' will be in phase whereby the grids permit a plate current flow.

In either of the embodiments of the invention shown or described, a hot cathode tube rt either the mercury vapor or .high vacuum type may be employed. In a three element high vacuum tube there is substantially no current flow in the grid circuit and therefore a life indicating circuit, if employed with such a circuit, must consist of an arrangement wherein the deterioration of the tube is indicated by means of ,providing an additional or fourth element in the tube. This fourth element may take the form of a separate element in the tube, and a life indicating circuit of this type for a high vacuum tube is shown in detail in my first above mentioned copending application. I

Another manner in which the present invention may be employed is disclosed in detail in the circuit shown in Figure l, wherein I have illustrated my invention as applied to the protection for a transformer upon the occurrence of a fault therein. In this embodiment of the invention, a power line I0 supplies power to th'e primary of a transformer through the usual switching connection including a circuit breaker I2. The secondary of the transformer supplies power to a feeder line I3. While I have illustrated only one phase of an alternating current system, it is obvious that the invention is applicable to a three phase system, the transformer in such case being a three phase transformer, but since the conditions for each phase and the circuits therefor are identical, only one phase has been illustrated. Y

For currrent transformation I may employ a current transformer indicated at 20, and since the design of commercial current transformers now in use is such as to require that the output side of the transformer be maintained substantially short circuited, I provide an additional special potential transformer for accomplishing this. 'This vspecial potential transformer is indicated at 2|, and has its primary connected to the output side of the respective current transformer through a low resistance overload relay 25. I preferably provide non-arclng condensers associated with each of the relays 22 and 25. It is desirable to introduce a time delay in the operation of the relay 25, and this may be done in any desired manner, as, for instance, by providing the relay with means within the relay for delaying its action, or with external means, either mechanical or electrical, for delaying the action. A conventional type of time delay for the relay is illustrated in my copending application referred to above. The secondary of the potential transformer 2| is connected from ground to the conductor through a series trip circuit relay 22 and through the tube 33 back to ground. It is apparent that the voltage induced in the secondary of the potential transformer 2l will vary in some relationship to the variations in the current flowing through the corresponding conductor l0.

The conductor 30 is connected to a plate element 32 within the control relay tube indicated generally at 33, and the tube is also provided with a cathode 34 adapted to be heated by a heater 35 and provided with a grid element 35.

It will be noted that the armature 23 controlled by the series relay coil 22 in the circuit of the special transformer is adapted, in energized position, to bridge across the conductor 26, to complete a circuit from the battery 21 and the conductor 25 thrrough the circuit breaker tripping coil 28, which is provided with an armature adapted, upon energization, to trip the circuit breaker I2. The circuit from the coil 20 extends through the conductor 29 and is completed through the armature 3| to the battery 21, the armature 3l I being controlled by the time delay or-overload cuited (normal operation).

relay 25 in the circuit of the current transformer. This relay 25 is adapted to operate after a predetermined time delay upon the occurrence of current flow therethrough in excess of the value for which .it has beenvset. It is apparent that both the relay 22 and the relay 25 must be operated in order to complete the circuit for tripping of the breaker l2.

In the relay arrangement described, there are three important factors to consider, irst,` theV tube operating characteristics, ,second, theelectrlcal characteristics of the special plate supply transformer, and lastly, the changes'in the phase angle of the line voltage 'with respect' to the ,bus voltage on fault conditions.

The special transformermus above system. As its primaryisccnncted inthe secondary circuit of the current:,vtransformer-it'y has to be designed so that it does' not place anfj unreasonable burden on the currenttransformer when its plate winding or secondary'is openclrnecessary plate voltage to actuate the tube when the 4current transformer has full load current and when the grid and plate are in phase. At heavy overloads the special transformer secondary voitage must be limited so as not to destroy the operating characteristics of the tube. Also, its open circuit voltage must be kept at a. minimum.

Considering now the grid circuit for the .tube I It must supply the struction substantially similar to that of the transformer 2|.

Under normal operating conditions the grid and plate voltages are approximately degrees out of phase, and hence no plate current flows through the tube 33. This particular embodiment of the invention is not intended to aord protection against reverse power flow, and will aiord no such protection due to the fact that upon a reversal of power ow, the relative phase relationship of the current on the primary side of the transformer to that on the secondary side will remain substantially the same in so far as concerns the vacuum tube relay 33. The apparatus is balanced so that under normal operating conditions the grid voltage and the y plate voltage are 180 degrees out of phase. Should a fault develop in either the primary or secondary side of the transformer, that is, within the transformer itself, the phase relationship of the current on the two sides of the transformer will be disturbed. In addition the magnitude of the current on the primary side will be increased while the magnitude of the current on the secondary side will be decreased. As a result of this change in relative phase relationship and as a further result of the change in magnitude of the current, the grid will permit current to flow through the plate circuit during a portion'of each cycle of the alternating current. Plate current controls the operation of the relay 22, resulting in the energization of the tripping coil 28 of the circuit breaker I2.

In the present instance I have employed in the system the time element overload relay 25, which may -be omitted if desired. This relay may be altered as to both the time element and as to the overload element, or if it is desired to retain the time delay elementthat renders operation of the circuit breaker independent of the overload, the relay 25 may be omitted and a time "element introduced into the operation of the relay 22.

rA suitable tube life indicating circuit may be employed in the circuit described in Figure 1, and will take the form of the circuit disclosed in detail in my copending application, Serial No. 638,513, nled October 19, 1932. y y

In Figure 4 I have disclosed aprotective arrangement for a generator or transformer, wherein'an unbalanced condition in the transformer, or

. f lf ultcausin anunb lanc inth' characteristics to operate satisfactorily *in 'they "anextema a g a e ecur rents in ,the-phase leads, which produces a flow of urrentf,v through the grounded neutral of the transformer vor, of a generator, results in operation ofthe controlrelayfor tripping the circuit breakcircuit.

There is provided a transformer or generator indicated generally at |00, having the three windings |02, |03 and |04, which are provided with a common neutral |05 connected to ground at |55` through the conductor |01. Normally, when the three phases of the transformer are balanced, there is no current ow at the point |05, and hence no current flow through the conductor |01.

However, when one of the phases or one of the legs of the transformer becomes unbalanced, a current now is set up at the point |05, and passes through the conductor |01 to ground. This induces a corresponding current now in a special 70 saturating transformer |00 connected to the conductor |01, which transformer may be of the type previously described, and which is provided with a saturating type reactor |09 connected across the t thereof. Reactor |09, may, if desired. 75

be replaced by a resistance, although in the preferred embodiment of the invention I prefer to use the reactor. The currentl from the saturating transformer passes through the reactor, and establishes a current flow through the resistance H and the lconductor H2 to the grid H3 of a control tube H4. The grid H3 is also connected, through the conductor I2, resistance' ||5, conductor H6 and the variable resistance H1 to the i has occurred in the transformer |00.

direct current operating bus H8. The cathode H9 of the tube is connected through the conductor |20 to a resistance |22 balanced between the negative bus H8 and the positive bus |23.

The tube H4 is also provided with a plate |24, which is connected through the conductor |25, and the relay |26 through the positive bus. Upon plate current flow from the plate |24 to the cathode H9, the relay |26 is energized to actuate the armature |21 to close the circuit across the contacts |28, which completes a circuit through the trip coil of a circuit breaker or the like for disconnecting the transformer from the power line.

Normally the grid H3 of the tube. H4 is provided with a negative bias from the bus |8, which bias prevents any operative plate current flow lfrom the plate |24 to the cathode H9. This negative bias may be of course'varied as desired by variation of the resistance H1.. Upon an unbalanced condition occurring in the transformer |00 resulting in current flow through the conductor |01, the transformer |08 is energized to pass a current through the resistance H0, which impresses a positive potential through the conduc- -tor ||2 upon the grid H3, overcoming this negative bias. 'I'his results in placing a positive potential on the grid, which allows operative plate current to flow from the direct current bus |23 through the coil |26 and the plate |24 to the cathode H9. Energization of the coil |26 due to this plate current ow results in tripping of the circuit breaker. The sensitivity of the control may be varied in accordance with the variation of the `connection of the conductor H6 to the resistance H1, so that the negative bias on the grid may be varied whereby a predetermined substantially instantaneous operation of the relay is effected upon an unbalanced condition occurring in the transformer.

In the circuit shown in Figure 5, I employ the principles of the circuit shown in Figure 4, but add thereto a time control element for delaying the operation of the tube H4 for a predetermined interval after an unbalanced condition Similar reference numerals indicate in Figure portions of the circuit which are common to Figures 4 and -5.

The unbalanced condition occurring in the transformer or generator |00 results in energization of the current transformer |08, which together with the saturating reactor |09, produces an alternating current which is led through the resistance |30 and condenser |3| to the plate |32 of a rectifying tube |33. The cathode `|34 of this tube is connected to the' opposite side of the circuit of the saturating transformer. `The alternating current transformer |08 therefore, in conjunction with the rectifying tube |33, produces a direct current positive potential which is transmitted from the plgte |32 through the conductor |35 and through the resistance |30 to the grid H3 of the tube H4. It will be noted that the grid H3 is normally negatively biased as described in connection with Figure 4. The positive direct current potential provided from the rectifier tube |33 may be controlled either by the variable resistance H0 or by the variable capacitance |36, so that a predetermined time interval must elapse between operation of the saturating 'current transformer |08 and the removal Aof the negative bias on the grid H3 of the tube H4. This time delay may be varied as desired, so that a certain predetermined interval is required for the positive direct current potential to build up to a sufficient extent to overcome the negative bias on the grid. Such a time delay circuit is broadly shown in my Patent No. 2,023,653, issued December l0, 1935.

The circuit for tripping the circuit breaker operates in the usual manner upon the application of a positive potential bias to the grid I3, namely, plate current flow is established from the .plate |24 to the cathode H9, resulting in energization of relay |26 and consequent tripping of the circuit breaker.

In Figure 6 I have shown a modified method of applying the protective system shown in Figures 4 and 5 to a transformer or generator to protect the system against faults developing internally of the transformer. In this particular application of the system, each of the transformer legs is separately protected by a differential relay conftrol system, so that a fault occurring on any one of the legs will serve to operate the circuit breaker.

Referring now in detail to Figure 6, the transformer |40 having the three windings |42, |43 and |44, is provided with a common neutral |45. Each of the legs of the transformer is provided with windings at opposite ends thereof indicated at |46 and |41, which are connected together by the conductors |48, and suitable taps from these conductors lead to the primary of a special saturating transformer |49.

Normally the windings |46 and |41 of each leg are in balanced condition, that is, the current ow therethrough is such that they balance each other so that there is no substantial current flow through the conductors |48. Upon a fault occurring in any of the transformer windings, an unbalanced condition is set up between the Windings |46 and |41 carried by the faulted leg of the transformer, and a current flow is set up in the conductors |48. resulting in current flow through the primary |49 of the saturating transformer. The secondary of this transformer, indicated at |50, is connected through the conductors |52 and |53 in a manner similar to that described in connection with the circuit shown in Figure 4. A suitable saturating reactor |54, corresponding to the reactor |09 of the circuit shown in Figure 4, is provided, and the remainder of the circuit is substantially the same as disclosed in connection with Figure 4, and similar reference numerals have been applied thereto to indicate corresponding parts. In the operation of the circuit, the occurrence of an unbalanced condition creates a current flow through the winding |50, which results in placing of a positive potential upon the grid H3, resulting in plate current flow from the plate |24 to the cathode H9 of the tube H4, thereby resulting in energization of the coil |26 for tripping the circuit breaker.

In Figure 8 I have disclosed a circuit corresponding to that shown in Figure 7, except for the addition of the timing elements H0' and |36, corresponding to the time delay effected in the circuit shown in Figure 5. In Figure 8, the occurrence of an unbalanced condition in one of the legs of the transformer |40 will result in flow of current to the rectifying tube E33, and in consequent i'low of direct current of positive potential to the grid H3 of the control relay. When the time controlling condenser |36, .or the resistance H' has been supplied with a sumcient direct current potential to supply the grid llt with. a positive potential overcoming the negative bias normally imposed thereon, the operative plate current flow is established resulting in energization of the relay |26 to operate the circuit breaker. The details oi?` the circuit shown in Figure 8 are the same as described in connection with Figures and 7, and-no further explanation is therefore deemed necessary.

In the modication shown in Figure 9, I have disclosed a transformer corresponding to that shown in Figure 6, and provided with the control windings about each of the legs thereof. However, instead of having three separate control circuits one connected to each of the windings |69, I provide for a conductor itt, which is connected by the conductors |62, |63 and |66 to the corresponding conductors its of the windings |46 and |41, and a conductor Ill! which is connected by the leads |12, il@ and H4 to the opposite conductor |88 of the windings |46 -and |47. The conductors i6@ and i'lilare then connected to the primary of' the saturating transformer |69, which transformer is common to all of the three legs of the transformer. The remainder of the circuit is similar to that de.- scribed in connection with either Figures 'l or 8, depending upon whether or not the time delay control is desired. Upon the occurrence of an unbalanced condition in any one of the legs |42, |43 or |44 of the transformer, a corresponding unbalanced condition will be set up resulting in current flow through the conductorsl i 6d and |10, resulting in current flow through the saturating transformer |49, and thus resulting in operation of the protective control circuit to operate the circuit breaker.

It is therefore believed apparent that I have provided a control or differential protection means operative to protect a circuit against in# ternal faults occurring within a transformer, whether by the use of a separate circuit for each of the transformer windings, or by the use amarsi of a circuit common to all of the transformer windings, whereby an unbalanced condition produced by a fault or disturbance in the transformer itself will result in operation of the circuit breaker. I have also provided for a dinerential protection system whereby a disturbance within a transformer will be controlled by the magnitude and phase relation of the currents on the primary and secondary sides of the transformer in, such manner as to operate a circuit breaker. The diierential protection means disclosed is also eiective for controlling the operation of circuit breakers for isolating a section of a line when a fault occurs within Vthe section'.

While various modicatlons and changes may be made within the disclosure of the present invention, the invention is not to be limited to the exact circuit shown and described, but only in so far as deiined by the scope and spirit of the appended claims.

I claim:

1. In a protective system for an electrical device having a plurality of phase windings, in combination, a pair of interconnected windings inductivelyrelated to the current flowing in each of said phase windings, electric valve means connected to be responsive" to unbalanced current flow between said pairs of windings and adapted to have the conductivity thereofchanged thereby, and relay means connected to said valve means and adapted to move from one operating position to another in response to said change in conductivity.

2. In a protective system for an electrical device having a plurality of phase windings, in combination, a pair of interconnected windings inductively related to the current owing in each of said phase windings, electric valve means common to all ofsaid pairs of windings and connected thereto in such manner as Jto beresponsive to unbalanced current flow between any pair of windings and to have the conductivity thereof changedthereby, and relay means connected to said valve means and adapted to move from one operating position to another in response to said change in conductivity.

ROBT. I. WARD. 

